zfscmd: fix crash in zfscmd_prometheus.go due to incorrectly extracted ProcessState

fixup of 96e188d7c4
refs #196
refs #301

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x10 pc=0x9a472a]

goroutine 15826 [running]:
os.(*ProcessState).systemTime(...)
        /home/cs/go1.13/src/os/exec_unix.go:98
os.(*ProcessState).SystemTime(...)
        /home/cs/go1.13/src/os/exec.go:141
github.com/zrepl/zrepl/zfs/zfscmd.waitPostPrometheus(0xc000c04800, 0xe21ce0, 0xc000068270, 0xbf9f80d88107e861, 0x19bae710e6, 0x13a8b60)
        /home/cs/zrepl/zrepl/zfs/zfscmd/zfscmd_prometheus.go:69 +0x22a
github.com/zrepl/zrepl/zfs/zfscmd.(*Cmd).waitPost(0xc000c04800, 0xe21ce0, 0xc000068270)
        /home/cs/zrepl/zrepl/zfs/zfscmd/zfscmd.go:155 +0x18a
github.com/zrepl/zrepl/zfs/zfscmd.(*Cmd).CombinedOutput(0xc000c04800, 0xc0004b8270, 0xd02eea, 0x3, 0xc0001f6c40, 0x3)
        /home/cs/zrepl/zrepl/zfs/zfscmd/zfscmd.go:40 +0xb3
github.com/zrepl/zrepl/zfs.ZFSRelease(0xe36aa0, 0xc0004b8270, 0xc0009a3a40, 0x13, 0xc0004a5d00, 0x1, 0x1, 0xed62eb221, 0x13a8b60)
        /home/cs/zrepl/zrepl/zfs/holds.go:102 +0x2a7
github.com/zrepl/zrepl/endpoint.ReleaseStep(0xe36aa0, 0xc0004b8270, 0xc0004befc0, 0xe, 0xd08482, 0x8, 0xc0001cb02f, 0x2, 0x1eeea3bff89dc90b, 0x134d6, ...)
        /home/cs/zrepl/zrepl/endpoint/endpoint_zfs_abstraction_step.go:130 +0x367
github.com/zrepl/zrepl/endpoint.(*Sender).SendCompleted.func2(0xc000459190, 0xc000390e30, 0xc00041fd80, 0xc0004befc0, 0xe, 0xd08482, 0x8, 0xc0001cb02f, 0x2, 0x1eeea3bff89dc90b, ...)
        /home/cs/zrepl/zrepl/endpoint/endpoint.go:419 +0x1c3
created by github.com/zrepl/zrepl/endpoint.(*Sender).SendCompleted
        /home/cs/zrepl/zrepl/endpoint/endpoint.go:413 +0x776

zfs/zfscmd/zfscmd.go

@@ -134,6 +134,10 @@ func (c *Cmd) waitPre() {
 	waitPreLogging(c, now)
 }
 
+type usage struct {
+	total_secs, system_secs, user_secs float64
+}
+
 func (c *Cmd) waitPost(err error) {
 	now := time.Now()
 
@@ -146,9 +150,34 @@ func (c *Cmd) waitPost(err error) {
 	c.waitReturnedAt = now
 	c.mtx.Unlock()
 
-	waitPostReport(c, now)
-	waitPostLogging(c, err, now)
-	waitPostPrometheus(c, err, now)
+	// build usage
+	var u usage
+	{
+		var s *os.ProcessState
+		if err == nil {
+			s = c.cmd.ProcessState
+		} else if ee, ok := err.(*exec.ExitError); ok {
+			s = ee.ProcessState
+		}
+
+		if s == nil {
+			u = usage{
+				total_secs:  c.Runtime().Seconds(),
+				system_secs: -1,
+				user_secs:   -1,
+			}
+		} else {
+			u = usage{
+				total_secs:  c.Runtime().Seconds(),
+				system_secs: s.SystemTime().Seconds(),
+				user_secs:   s.UserTime().Seconds(),
+			}
+		}
+	}
+
+	waitPostReport(c, u, now)
+	waitPostLogging(c, u, err, now)
+	waitPostPrometheus(c, u, err, now)
 }
 
 // returns 0 if the command did not yet finish

zfs/zfscmd/zfscmd_logging.go

@@ -1,7 +1,6 @@
 package zfscmd
 
 import (
-	"os/exec"
 	"time"
 )
 
@@ -28,23 +27,12 @@ func waitPreLogging(c *Cmd, now time.Time) {
 	c.log().Debug("start waiting")
 }
 
-func waitPostLogging(c *Cmd, err error, now time.Time) {
-
-	var total, system, user float64
-
-	total = c.Runtime().Seconds()
-	if ee, ok := err.(*exec.ExitError); ok {
-		system = ee.ProcessState.SystemTime().Seconds()
-		user = ee.ProcessState.UserTime().Seconds()
-	} else {
-		system = -1
-		user = -1
-	}
+func waitPostLogging(c *Cmd, u usage, err error, now time.Time) {
 
 	log := c.log().
-		WithField("total_time_s", total).
-		WithField("systemtime_s", system).
-		WithField("usertime_s", user)
+		WithField("total_time_s", u.total_secs).
+		WithField("systemtime_s", u.system_secs).
+		WithField("usertime_s", u.user_secs)
 
 	if err == nil {
 		log.Info("command exited without error")

zfs/zfscmd/zfscmd_prometheus.go

@@ -46,7 +46,7 @@ func RegisterMetrics(r prometheus.Registerer) {
 	r.MustRegister(metrics.usertime)
 }
 
-func waitPostPrometheus(c *Cmd, err error, now time.Time) {
+func waitPostPrometheus(c *Cmd, u usage, err error, now time.Time) {
 
 	if len(c.cmd.Args) < 2 {
 		getLogger(c.ctx).WithField("args", c.cmd.Args).
@@ -64,10 +64,10 @@ func waitPostPrometheus(c *Cmd, err error, now time.Time) {
 
 	metrics.totaltime.
 		WithLabelValues(labelValues...).
-		Observe(c.Runtime().Seconds())
+		Observe(u.total_secs)
 	metrics.systemtime.WithLabelValues(labelValues...).
-		Observe(c.cmd.ProcessState.SystemTime().Seconds())
+		Observe(u.system_secs)
 	metrics.usertime.WithLabelValues(labelValues...).
-		Observe(c.cmd.ProcessState.UserTime().Seconds())
+		Observe(u.user_secs)
 
 }

zfs/zfscmd/zfscmd_report.go

@@ -57,7 +57,7 @@ func startPostReport(c *Cmd, err error, now time.Time) {
 	active.mtx.Unlock()
 }
 
-func waitPostReport(c *Cmd, now time.Time) {
+func waitPostReport(c *Cmd, _ usage, now time.Time) {
 	active.mtx.Lock()
 	defer active.mtx.Unlock()
 	prev := active.cmds[c]